Lean-burn engines have been widely used to improve vehicle fuel economy. As exhaust emission regulations have become more stringent, a lean nitrogen oxide trap (LNT) is typically mounted onto the rear end of an exhaust manifold, and a diesel particulate filter (DPF) is mounted onto the rear end of the LNT in vehicles in which a lean-burn engine is mounted so as to reduce the quantity of toxic exhaust gases emitted from the engine.
The LNT serves to trap and store nitrogen oxides (NOx) generated due to lean burning in the engine, reduce the NOx into nitrogen (N2) by means of a reduction reaction, and then discharge the nitrogen (N2). Such an LNT may be subjected to sulfur poisoning, which occurs due to sulfur components included in fuel and engine oil, and thus the ability to trap NOx may be deteriorated. In such cases, the NOx purification performance must be restored through desulfurization.
Candidate systems used to solve the above problems include LNTs and passive selective catalytic reduction (pSCR) systems. The LNTs and pSCR systems serve to store NOx in an LNT in a general driving mode, in which oxygen is plentiful. When NOx is present in an amount greater than a predetermined amount, the driving mode is converted to an enriched mode, in which the fuel in an engine is enriched and NOx stored in the LNT is converted into harmless N2, which is then removed. In this case, some of the NOx stored in the LNT is converted into NH3 at the LNT, and the NH3 is stored in an SCR unit, located downstream of the LNT. Then, once the driving mode is converted from the enriched mode back to a general driving mode, in which oxygen is plentiful, NH3 reacts with NOx that slipped through the LNT to generate N2 which is then removed.
The removal of NOx from the LNT takes place as follows: NO+CO→CO2+½N2, where one molecule of NOx is removed. The removal of NOx from a pSCR unit takes place as follows: NO+5/2H2→NH3+H2O and NH3+NO+WO2→N2+3/2H2O, where two molecules of NOx are removed.
In the prior art, an enriched mode for purifying stored NOx is terminated when values measured at lambda sensors installed at the front/rear ends of the LNT are identical to each other. The lambda value measured at the rear lambda sensor is maintained high as oxygen and NOx present in the LNT are detached, but becomes identical to a lambda value measured at the front lambda sensor when these chemical species are completely consumed in the LNT. In this case, it is judged that the regeneration of NOx in the LNT is complete.
However, since the lambda sensors are highly affected by O2 detachment, NOx may remain in the LNT even after termination of the enriched mode when the lambda sensors are applied to conventional control systems. Such residual NOx may be converted into NH3, and NH3 generation may be activated since no oxygen is present in a subsequent enriched mode. Therefore, it is necessary to minimize residual NOx in order to improve the performance of the LNT and the pSCR unit. Accordingly, there is a need for a control method for improving NOx purification performance capable of increasing the quantity of O2 that is emitted without installing additional equipment, and capable of minimizing residual NOx and increasing NH3 generation by delaying the point in time at which lambda values measured at the front/rear lambda sensors are found to be the same so as to delay the time at which NOx is regenerated in an enriched mode.
The contents described in the prior art are merely illustrated to aid in understanding the background of the present disclosure, and thus it should be understood that the contents are not deemed to fall within the prior art already known by those skilled in the related art.